Which paper introduced the term "softmax"?
The paper that appears to have introduced the term &quot;softmax&quot; is Training Stochastic Model Recognition Algorithms as Networks can Lead to Maximum Mutual Information Estimation of Parameters (1989, NIPS) by John S. Bridle.
As a side note, the softmax function (with base $b = e^{-\beta}$)
$$\sigma (\mathbf {z} )_{i}={\frac {e^{-\beta z_{i}}}{\sum _{j=1}^{K}e^{-\beta z_{j}}}}{\text{ for }}i=1,\dotsc ,K {\text{ and }}\mathbf {z} =(z_{1},\dotsc ,z_{K})\in \mathbb {R} ^{K}$$
is very similar to the Boltzmann (or Gibbs) distribution
$$
p_i=\frac{e^{- {\varepsilon}_i / k T}}{\sum_{j=1}^{M}{e^{- {\varepsilon}_j / k T}}}
$$
which was formulated by Ludwig Boltzmann in 1868, so the idea and formulation of the softmax function is quite old.